ABSTRACT
The most striking feature of glass-forming liquids, either simple or polymeric liquids, is a steep increase of their viscosity as temperature decreases [1]. Usually, the glass transition temperature Tg is arbitrarily defined as the temperature at which the viscosity reaches 1012 Pa.s in simple liquids, or at which the dominant relaxation time τα becomes larger than a macroscopic time scale, for example, τg ~ 1 s or 102 s [2]. Another important feature of glass transition is the strongly heterogeneous nature of the dynamics close to Tg [3,4]. This strongly heterogeneous nature has been demonstrated experimentally over the past 20 years using nuclear magnetic resonance (NMR) [5-7], fluorescence recovery after photobleaching (FRAP) [8-12], dielectric hole burning [13], or solvation dynamics [14]. These studies have demonstrated the coexistence of domains with relaxation time distributions spread over more than four decades at temperatures typically 20 K above Tg. The diffusion dynamics of molecular probes about 1 nm in diameter has also been measured, showing that the Stokes’ law is essentially valid in the high-temperature range, but breaks down below Tg + 20 K [8]. This was one of the earlier indications of the spatial nature of dynamical heterogeneities and provided an estimate of their size ξ. The characteristic size ξ has been estimated by NMR [6] to be 3-4 nm at Tg + 20 K (in the case of van der Waals liquids), whereas it is as small as 1 nm in glycerol [7].
